The broad, long-term objectives of the project are to characterize the structure, regulation and function of the protein tyrosine phosphatase (PTP) family of enzymes. It is now apparent that the coordinated and competing actions of both protein tyrosine kinases (PTKs) and PTPs are integrated in vivo to control such fundamental processes as growth and proliferation, differentiation, survival, motility and metabolism. Furthermore, disruption of the delicate balance between the action of PTPs and PTKs has been implicated in the etiology of human diseases, including cancer, diabetes and inflammation. Therefore, characterization of the PTPs is a prerequisite to gaining a complete understanding of the physiological consequences of tyrosine phosphorylation and how such signaling events are abrogated in disease. The focus of this proposal is the regulation of PTP function by reversible oxidation and inactivation, as a new tier of control of tyrosine phosphorylation-dependent signaling. In the previous funding period, a novel assay was developed, and extensively validated, that measures the reversible oxidation in cells of all the major categories of the PTP family. In this competitive renewal various tools that have been developed in the lab will be applied to examine the role of PTP oxidation in cancer models. In addition, the mechanisms underlying the reduction step in the process of reversible oxidation, which regenerates the active form of the PTP, but which has been neglected to date, will be characterized. It is anticipated that these studies will generate new insights into the regulation of tyrosine phosphorylation-dependent signal transduction, as well as new insights into the function of individual PTPs in cancer. To achieve this, the Specific Aims of the research program are: 1: To investigate the regulation and function of the receptor phosphatase PTP1 in cell and animal models of breast cancer, 2: To investigate the role of PTP oxidation in onset and bypass of oncogene-induced senescence, 3: To investigate mechanisms regulating the reduction and reactivation of oxidized PTP1B, and 4: To investigate the role of thioredoxin in reversible PTP oxidation.